Photoresponsive devices based on semiconductive polymers have been investigated for a number of years, examples of such devices can be found in WO 99/09603, GB 2,315,594, U.S. Pat. No. 5,523,555, and U.S. Pat. No. 5,670,791. In these disclosures polymer photoresponsive devices include an active polymer layer or layers situated between two electrodes. Where the two electrodes have different work functions a built-in, internal field is generated across the active polymer layer giving rise to a photovoltaic effect, such devices are termed photovoltaic cells. A device of this construction will also have rectifying properties so may also be termed a photodiode. Photoresponsive devices have application as solar cells which generate electricity from light and as photodetectors which measure or detect light.
Photovoltaic devices such as that described in U.S. Pat. No. 5,670,791 have a construction in which a layer comprising a blend of two semiconductive polymers having different electron affinities and/or ionization potentials is situated between two electrodes of different work function, FIG. 1 shows such a device having a transparent glass substrate (1), a transparent anode (2), a hole transport layer (3), a layer comprising a blend of two semiconductive polymers forming an interpenetrating network (4) and a metallic cathode (5). The electrodes of different work function set up an internal electric field across the device. Such devices operate on the principle that absorption of light by the polymers of the organic layer generates bound electron-hole pairs, termed excitons. Excitons generated on the polymer of lower electron affinity dissociate by transfer of an electron to the polymer of higher electron affinity, the polymer of lower electron affinity is sometimes referred to as the electron donor or simply donor. Excitons generated on the polymer of higher electron affinity dissociate by transfer of a hole to the polymer of lower electron affinity, the polymer of higher electron affinity is sometimes referred to as the electron acceptor or simply acceptor. The electrons and holes generated by dissociation of the excitons then move through the device, with electrons moving to the lower work function cathode and holes moving to the higher work function anode. This is shown in FIG. 2(a) which shows a photovoltaic device having an interpenetrating network of two semiconductive polymeric materials one of which has a higher electron affinity (41) than the other (42), FIG. 2(b) shows schematically an exciton (43) generated by an incident photon being split into an electron (44) and a hole (45), the electron moving through the polymer of higher electron affinity (41) and the hole moving through the polymer of lower electron affinity (42). In this way light incident on the device generates a current which may be used in an external circuit. Photoresponsive devices may also be constructed in which the electrodes are of the same work function and an external bias is applied across the device to fulfill the function of the above described internal bias.
Increasing the efficiency of photovoltaic devices is a primary concern in the solar cell industry. At present photovoltaic cells based on organic materials are characterised by low power conversion efficiencies and low external quantum efficiencies. The power conversion efficiency is the ratio of the power taken out of the device in the form of electricity to the power put into the device in the form of incident light. The external quantum efficiency is the ratio of electrons collected from a device to the number of photons incident on the device, this is also known as the quantum yield.
In WO 99/49525 a method for increasing the efficiency of a photovoltaic device is disclosed in which a component having an electrode and a layer of a semiconductive polymer which acts as an electron donor and component having an electrode and a layer of a semiconductive polymer which acts as an electron acceptor are laminated together in such a manner so as to form a device with a layer comprising a mixture of the two semiconductive polymers at the interface where the two components are laminated together. A device prepared in this way was found to have higher efficiency than a device prepared from a blend of two semiconductive polymers, such as the device disclosed in the aforementioned U.S. Pat. No. 5,670,791.
It has been observed in photovoltaic devices having a double layer structure in which the layers comprise the semiconductive polymer poly(3-dodecylthiophene) and the semiconductive molecule N,N′-diphenylglyoxaline-3,4,9,10-perylene tetracarboxylic acid diacidamide or a mixture of the two, that annealing the double-layer structure device leads to improvements in device efficiency, see Feng et al, Journal of Applied Physics, 88 (12), 2000, 7120-7123.
Similarly a photovoltaic device comprising a blend of poly(phenylenevinylene) and a perylene derivative has been prepared, annealing the device was shown to increase the external quantum efficiency of the device, see Dittmer et al, Solar Energy Materials and Solar Cells, 61, (2000), 53-61.